Sergeant Jason Reyes of the Texas Department of Public Safety, which is co-ordinating what he called a "search and recovery" effort rather than the previous day's "rescue", also confirmed that the number of reported injuries had risen from about 170 to "approximately 200". The official number of fatalities, however, appeared to contradict a claim from the mayor of West, Tommy Muska, who told reporters on Thursday that "around 35 people" were dead.

"I cannot speak on behalf of the mayor," Reyes said at a press conference early on Friday. "I have not talked to him today. We have left messages for him."

The bodies, mostly of firefighters who were tackling a blaze at the West Fertilizer Company when a store of ammonia-based chemicals ignited, were taken to the Dallas forensic laboratory for identification, according to Reyes.

"It is with a heavy heart that I can confirm 12 individuals have been recovered from the fertilizer plant explosion. We know these bodies were in the area of the explosion," he said. "We continue with the recovery effort. We are still treating this as a crime scene, we will go in there and make sure we conduct a thorough investigation." He later updated the number of deaths to 14.

Although officials are not yet confirming identities, some of those known to have died were volunteer members of the West fire department, a 29-strong force whose members rushed to the plant after receiving a 911 call reporting a fire at about 7.30pm on Wednesday, about 25 minutes before the explosion. Local media named Jerry Chapman, 26, as one of the victims. In Dallas, mayor Mike Rawlings paid tribute to Kenny Harris, a 52-year-old father of three and a captain in the city's fire department, who was not on duty in West but who was killed after he went to assist the volunteers.

Officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), assisted by the US Chemical Safety Board, have begun their inquiry into the cause of the blast, which sent a fireball almost 100 metres into the air and left a giant crater where the plant had once been. The inquiry could take six months, an ATF spokeswoman said. Investigators will focus on which chemicals were stored at the plant. The West Fertilizer Company was licensed for 54,000lbs of anhydrous ammonia, a liquid nitrogen agricultural fertilizer that is stored in large reinforced steel tanks.

Officials at first suggested the explosion was caused by the anhydrous ammonia igniting, but it was revealed Thursday through Texas state records that the plant also possessed 270 tons of ammonium nitrate, a much more volatile, dry solid, at the end of 2012. Records also suggested that in 2006, the Environmental Protection Agency (EPA) issued a $2,300 fine for deficiencies in the plant's risk management plan.

According to data from the Texas Commission on Environmental Quality and the EPA, the last documented regulatory visit to the plant took place in late 2007, as a follow-up to a complaint of an odour coming from the site. The Occupational Safety and Health Administration – a body tasked with making sure America's 7 million workplaces are safe for employees – has not made a site visit since 1985.

Greg Abbott, the Texas state attorney, said on Thursday it was too early to say if anybody might be held criminally negligent. The owner of West Fertilizer, Don Adair, has not spoken publicly since the incident, although officials say he is co-operating with inquiries.

Some of the 2,800 residents of the town, many of whom were evacuated on Wednesday night over fears of a second explosion, were allowed to return to their homes on Friday, to begin repairs or to salvage possessions if the buildings were too badly damaged. Many of those displaced are staying with friends and family, in local hotels or at a number of shelters set up by the American Red Cross and other charities.

"The reality of what happened will begin to set in after a day or two as people get back to their homes, and many will need support to help them through the grieving process," said Captain Rojelio Galabeas, the head of a team of Salvation Army volunteers from San Antonio.